Abrasive product

ABSTRACT

An abrasive product which comprises a polycrystalline mass of self-bonded abrasive particles of irregular shape, the product being substantially free of a second phase and containing substantial plastic deformation of the abrasive particles. The abrasive particles are preferably diamond or cubic boron nitride and the plastic deformation of the particles is preferably at least 0.3 percent. The abrasive product may be made by subjecting a mass of the particles to elevated temperature and pressure conditions.

BACKGROUND OF THE INVENTION

[0001] This invention relates to an abrasive product comprisingpolycrystalline self-bonded abrasive particles, and to a method ofmaking such a product.

[0002] The production of leached self-bonded polycrystalline particlesis taught in U.S. Pat. No. 4,776,861 in the name of General ElectricCompany. The method of this reference involves making a polycrystallinecompact according to the teaching of, for example U.S. Pat. Nos.3,141,746, 3,745,623, 3,767,371, 4,104,344, 3,609,818 and 4,224,380,size reducing the compact, e.g. by crushing, and leaching thenon-particle matter such as solvent/catalyst from the bonded particles.

[0003] The leaching of the non-particle matter from the bonded particlesbecomes increasingly difficult as the grain size of the crystals makingup the particles decreases, as the porosity of the self-bonded particlesdecreases, and as the size of the self-bonded particles increases.

[0004] The self-bonded particles produced according to the method ofU.S. Pat. No. 4,776,861 are characterised by a large extent ofdiamond-to-diamond bonding, which results in a low friability (highstrength), and a low porosity, usually less than about 10% .

[0005] Polycrystalline self-bonded diamond particles may also beproduced using a shock wave method as taught in U.S. Pat. No. 3,238,019.The method involves non-diamond carbon being mixed with an inertmaterial, and subjecting the mixture to an explosive shock wave in whichvery high pressures and high temperatures are realised for a short timeperiod. The inert material is included in the mixture to facilitate heatremoval from the system. The mixture is recovered from the explosion andcrushed, before chemical treatment to remove remaining non-diamondcarbon and the inert material. The cleaned diamond is then crushedfurther and graded as necessary.

[0006] Polycrystalline particles produced according to the method ofU.S. Pat. No. 3,238,019 are characterised by a high friability (lowstrength) and very limited self-bonding.

[0007] U.S. Pat. No. 4,181,505 discloses a method of producing freediscrete work-hardened diamond crystals. A large diamond is embedded ina mass of relatively large diamond crystals and this assembly subjectedto temperature and pressure conditions at which diamond isthermodynamically stable. The large diamond crystal is work-hardened inthis manner and is said to have shown evidence of plastic deformation.

SUMMARY OF THE INVENTION

[0008] According to a first aspect of the invention, there is providedan abrasive product comprising a polycrystalline mass of self-bondedabrasive particles of irregular shape, the product being substantiallyfree of second phase and containing substantial deformation of theabrasive particles.

[0009] The abrasive product is a polycrystalline mass of self-bondedabrasive particles and is substantially free of a second phase oradditional component.

[0010] In other words, there is no bonding metal or sintering aid suchas a solvent/catalyst, or other such material. Any additional componentwhich may be present is there in trace amounts only.

[0011] Further, the diamond particles are of irregular shape. They may,for example, be particles which have been produced by crushing ormilling action. Thus, in the self-bonded product, there is evidence ofasperities, sharp points or edges of one crystal bearing upon asubstantially flat area of an adjacent crystal, resulting in plasticdeformation at the contact points between crystals. The plasticdeformation of the particles is preferably at least 0.3 percent, andmore preferably at least 0.5 percent.

[0012] The method of measuring plastic deformation, as used in thespecification is as follows:

[0013] Step 1

[0014] The shapes (profiles) of the X-ray diffraction peaks, viz, (100),(220), (311) and (331), for the diamond before treatment are recorded.

[0015] Step 2

[0016] The shapes of the same X-ray diffraction peaks are recorded aftertreatment, using a sample that has been crushed to less than about 120microns, and using the same X-ray diffraction conditions.

[0017] Step 3

[0018] The widths of corresponding before and after peaks are compared,and the widths of the peaks at half their maximum intensities aremeasured using a curve fitting software routine. Simultaneously, thepeaks are deconvoluted into their Kα₁ and Kα₂ components.

[0019] Each peak is in fact a combination of two peaks due to Kα₁ andKα₂ radiations. Both peaks have the same shape and are related to oneanother in terms of intensity (Kα₁ being about twice the intensity ofKα₂) and relative peak maximum positions (the difference in positionsincreases with increasing Bragg angle in a predetermined way).

[0020] Step 4

[0021] The broadening for each pair of before and after treatment peaksis calculated. This calculation may be done in several ways. The wayused in this specification is:

β² =B ² −b ²

[0022] where β in the peak broadening due to plastic deformation and/orcrystallite size, B is the measured peak breadth after treatment and bis the measured peak breadth before treatment. For large differences,the method of calculation of β has little effect on the result.

[0023] Step 5

[0024] The value of β. cos θ for each peak is calculated and plottedagainst the value sin θ.

[0025] Step 6

[0026] The slope of the least squares line of best fit is calculated.This slope is indicative of the plastic deformation.

[0027] The definitive part of the method used is the equation, β. cosθ=Kλ/L+η.sin θ. To a person skilled in the art of X-ray diffraction,this conveys the theoretical and analytical principles employed in themethod of determining the plastic deformation, η.

[0028] The abrasive product will also generally have a porosity of atleast 5 percent by volume, and preferably a porosity which is greaterthan 10 percent by volume and does not exceed 25 percent by volume.

[0029] The abrasive particles may be diamond, cubic boron nitride (cBN),tungsten carbide, silicon carbide, quartz or corundum. The preferredabrasive particles are ultra-hard abrasive particles such as diamond orcubic boron nitride. In the case of diamond, the diamond may besynthetic diamond made by a high pressure-high temperature (HPHT)process, synthetic diamond made by a chemical vapour deposition (CVD)method, shock wave diamond or natural diamond.

[0030] According to a further aspect of the invention, there is provideda method of making an abrasive product as described above which includesthe steps of:

[0031] (a) providing a mass of abrasive particles,

[0032] (b) subjecting the mass of particles to conditions of elevatedtemperature and pressure to self-bond the particles together in theabsence of a second phase. Thus, the mass of particles is substantiallyfree of any additional component or second phase at the time it issubjected to the elevated temperature and pressure conditions.

[0033] The conditions of elevated temperature and pressure which areapplied will depend upon the nature of the abrasive particles used. Inthe case of ultra-hard abrasive particles, these conditions of elevatedtemperature and pressure are preferably such that the ultra-hardabrasive particle is thermodynamically stable.

[0034] In the case of diamond, the conditions of temperature andpressure will generally be in the region of diamond thermodynamicstability in the graphite-diamond phase diagram. Conditions outside theregion of diamond thermodynamic stability may also be used provided thatthe time during which the conditions are applied is insufficient forsignificant reversion of the diamond to graphite to take place.Typically, the conditions used are temperatures in the range 750 to1400° C. and pressure in the range 3 to 6 GPa.

[0035] In the case of cubic boron nitride, the conditions of temperatureand pressure will generally be in the region of cubic boron nitridestability in the boron-nitrogen phase diagram. Conditions outside theregion of cubic boron nitride stability may also be used provided thatthe time for which the conditions are applied is insufficient forsignificant reversion of the cubic boron nitride to hexagonal boronnitride to take place. Typically, the conditions used are temperaturesin the range 750 to 1400° C. and pressures in the range 3 to 6 GPa.

[0036] In one preferred form of the invention, the abrasive particlesare ultra-hard abrasive particles such as diamond or cBN and in step(b), the pressure is first raised to above 1 GPa, the temperature thenraised to bring the particles into a temperature and pressure region toproduce plastic deformation of the particles and the pressure thereafterraised to bring the particles into a region in which the particles arethermodynamically stable.

[0037] The particles are of irregular shape and thus have asperities,points and edges as well as some flat areas. It is believed thatself-bonding of the particles occurs due to the very high contactpressure generated when an asperity, point or edge on one particle bearsupon a substantially flat area of an adjacent particle. The very highcontact pressure is well in excess of the nominal applied pressure ofthe pressurising system. Such high contact pressure when applied at anelevated temperature causes plastic deformation at the contact pointsbetween particles thereby promoting the movement of the constituentatoms of the crystal and facilitating self-bonding. Generally, theextent of self-bonding is determined by the selected conditions oftemperature and pressure and the period for which the conditions areapplied. The extent of self-bonding of the particles determines thestrength or friability of the polycrystalline bonded product.

[0038] The abrasive product which is produced by the method describedabove is relatively large and will generally have a largest dimension ofat least 1 mm. The product may have a variety of shapes such ascylindrical and may be used as such for abrasive operations such as inbruting tools or as blanks for wire drawing dies. By way of example,cylinders having a diameter of up to 75 mm and thicknesses of up to 20mm may be produced. The thus produced abrasive product may be sizereduced, for example by cutting to smaller pieces millimetres in size.The size reduction may also be achieved by crushing or milling toproduce smaller polycrystalline self-bonded products or particlesgenerally having a particle size of less than 500 microns and morepreferably less than 100 microns. The crushed product may be used inpolishing or lapping, either dry or as a slurry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a pressure/temperature graph illustrating an example ofa temperature-pressure profile for use in the method of the invention;and

[0040]FIG. 2 is a photomicrograph of a polished surface of apolycrystalline self-bonded abrasive product of the invention.

DESCRIPTION OF EMBODIMENTS

[0041] The abrasive product of the invention is characterised by being apolycrystalline mass of self-bonded abrasive particles of irregularshape and by the product being substantially free of second phase andcontaining substantial plastic deformation of the abrasive particles.Further, it has been found that the product, particularly when theparticles are ultra-hard abrasive particles, has a friability betweenthat of shockwave diamond particles such as that produced by the methodof U.S. Pat. No. 3,238,019 and leached polycrystalline diamond particlesas produced, for example, by the method described in U.S. Pat. No.4,776,861.

[0042] In carrying out the method of the invention, with ultra-hardabrasive particles, it is preferred that the particles are below adefined maximum. By this is meant that generally at least 80 percent,and preferably at least 90 percent, do not exceed the defined maximumparticle size.

[0043] In the case of diamond particles, the defined maximum particlesize is typically 60 microns, preferably 50 microns, with a lower limitof particle size of about 0.1 microns. In the case of cubic boronnitride particles, the defined maximum particle size is typically 500microns, preferably 200 microns, with a lower limit of particle size ofabout 0.1 microns. This lower limit is imposed by the limitations ofcrushing and grading methods and not by the method of the invention.

[0044] When the particles are ultra-hard abrasive particles thetemperature-pressure conditions which are applied are preferably thoseillustrated in the attached FIG. 1. Referring to FIG. 1, the temperatureand pressure conditions applied or profile is illustrated by line 10. Itwill be noted that the pressure is first raised to a value above 1 GPa,and the temperature thereafter raised to a point A at which theparticles are placed in a region C which is a region of plasticdeformation of the particle. The region C is bounded by line 12. Line 12of FIG. 1 exemplifies but does not define the boundary between theregion of plastic deformation and the region of no plastic deformation.It is a transition region and not a sharp cut-off. Furthermore, thedisplacement of the line 12 to higher or lower temperatures depends uponthe nature of the impurities in the abrasive particle (e.g. boron ornitrogen) and the level of those impurities. Thereafter, the pressure isagain raised, for example, to a point B which is above line 14, thethermodynamic equilibrium line for the particular ultra-hard abrasiveparticle used. Pressures below the equilibrium line 14 can be usedprovided that the time for which the conditions are maintained isinsufficient for significant transformation of the ultra-hard abrasiveto take place.

[0045] The particles will generally be placed in a container whichitself is placed in the reaction zone of a known high pressure/hightemperature apparatus for application of the required conditions oftemperature and pressure. The bonded product may be recovered from thecontainer in any known manner, for example by mechanical removal orchemical digestion of the container.

[0046] The properties of the abrasive product of the invention may bemodified by infiltrating the particles with a desired infiltrant such asa metal, alloy, plastic, ceramic or glass, or precursors of any of theseinfiltrants.

[0047] Optionally, the abrasive product of the invention may be cladwith a suitable coating, such as titanium or copper.

[0048] The invention is illustrated by the following examples.

EXAMPLE 1

[0049] A quantity of diamond powder was made by crushing and gradinghigh pressure-high temperature (HPHT) synthetic diamond crystals to aparticle size range of 3 to 5 microns. The powder was washed with acidto remove contamination, rinsed with de-ionised water and dried. Thediamond was of an irregular shape. A 50 g quantity of the cleaneddiamond was placed in a titanium metal canister, and the canister wasplaced in a high pressure, high temperature apparatus. The canister wasraised to conditions of about 5 GPa and about 1200° C., using atemperature-pressure profile similar to that shown in FIG. 1, and theseconditions were maintained for 30 minutes. After this treatment, thecanister was recovered from the reaction volume, and the treated diamondliberated by mechanically removing the embrittled titanium with ascalpel. The diamond had formed a coherent compact with a porosity ofabout 22%, estimated from the mass and dimensions of the compact.

[0050] The compact was crushed using a pestle and mortar and theresulting particles screened to a particle size range of 88 μm to 44 μm.A number of particles were mounted in resin and polished. The polishedsections of polycrystalline self-bonded diamond particles weresubstantially as shown in FIG. 2, with distinct areas displaying“necking” and self-bonding pointed out by the arrows. A sample of thepotycrystalline self-bonded particles was crushed further to passthrough a 44 μm screen, and examined using an X-ray diffractometer. Fromthe broadening of the (100), (220), (311) and (331) reflexions, theplastic deformation of the diamond crystals was measured to be about1.1%.

EXAMPLE 2

[0051] A polycrystalline self-bonded diamond particle product was madeaccording to the procedure described in Example 1 using diamond crystalsof irregular shape with a nominal size range of 0.5 to 1 micron, andusing the same temperature-pressure conditions. The self-bonded massformed a coherent compact with a porosity of about 9%. The compact wascrushed and examined in the same manner as Example 1. Only carbon wasfound by X-ray mapping of the polished section. The plastic deformationwas measured to be 0.9%.

EXAMPLES 3 to 8

[0052] In these examples, the temperature-pressure conditions andexamination methods as used in Example 1 were applied to diamondcrystals of different size ranges and different origins. Example 7 useda similar temperature-pressure profile except the final temperature wasabout 1400° C. Example 8 used a similar temperature-pressure profileexcept the final pressure was about 3.0 GPa. All self-bonded diamondparticle masses formed coherent compacts after step (b), and all showedself-bonding of the diamond crystals, with a general appearance as inFIG. 2 Diamond Diamond Plastic deformation Porosity Example type size(%) (%) 3 HPHT less than 0.5 0.7 5 synthetic micron 4 HPHT 30 to 40 0.720 synthetic microns 5 Natural  8 to 16 0.9 10 microns 6 CVD 49 to 570.65 14 microns 7 HPHT 3 to 5 1.1 n.d. synthetic microns 8 HPHT 3 to 50.9 n.d. synthetic microns

EXAMPLE 9

[0053] A quantity of cubic boron nitride crystals with a nominal size of3 microns was subjected to the temperature-pressure conditions ofExample 1 except that the final temperature was about 1100° C. and thenexamined as set out in Example 1. The self-bonded mass after step (b)was coherent. The plastic deformation was measured to be 1.15%.

EXAMPLE 10

[0054] The quantitative measurement of friability of weakpolycrystalline particles is unreliable. A simple comparative evaluationof the friabilities of a selection of polycrystalline self-bondedparticles, a sample of shock wave particles and a sample of leachedpolycrystalline diamond produced according to the teachings of U.S. Pat.No. 4,776,861 was undertaken. A small quantity of each sample, withnominally the same particle size range were placed on a steel plate, andsqueezed and crushed with a rotating motion using a spatula. The sampleswere then ranked with respect to the ease with which the particles werebroken. The ranking in order of decreasing friability was as follows:Rank Sample Comments 1 Shock wave Smeared over the plate Diamond surfacevery easily, highly friable 2 Example 9 Broke under low pressure, didnot smear, less friable than shock wave particles 3 Example 1 Brokeunder moderate pressure, less friable than Example 9 4 Example 8 Brokeonly under the highest pressure, more friable than leached particles 5Leached Did not break under highest polycrystalline pressure, notfriable

1. An abrasive product comprising a polycrystalline mass of self-bondedultra-hard abrasive particles of irregular shape, the product beingsubstantially free of a second phase, containing substantial plasticdeformation of the abrasive particles, such plastic deformation being atleast 0.3 percent, and having a porosity which is greater than 10percent by volume and does not exceed 25 percent by volume.
 2. Anabrasive product according to claim 1 wherein the plastic deformation ofthe particles is at least 0.5 percent.
 3. An abrasive product accordingto claim 1 or claim 2 wherein the ultra-hard abrasive particles arediamond particles.
 4. An abrasive product according to claim 3 whereinthe particle size of the diamond particles does not exceed 60 microns.5. An abrasive product according to claim 3 wherein the particle size ofthe diamond particles does not exceed 50 microns.
 6. An abrasive productaccording to claim 1 or claim 2 wherein the abrasive particles are cBNparticles.
 7. An abrasive product according to claim 6 wherein theparticle size of the cBN particles does not exceed 500 microns.
 8. Anabrasive product according to claim 6 wherein the particle size of thecBN particles does not exceed 200 microns.
 9. An abrasive productaccording to any one of the preceding claims which has a largestdimension of at least 1 mm.
 10. An abrasive product according to any oneof claims 1 to 8 having a size of less than 500 microns.
 11. A method ofmaking an abrasive product according to any one of the preceding claimsincluding the steps of: (a) providing a mass of ultra-hard abrasiveparticles, (b) subjecting the mass of particles to conditions ofelevated temperature and pressure at which the ultra-hard abrasiveparticle is thermodynamically stable to self-bond the particles togetherin the absence of a second phase.
 12. A method according to claim 11wherein in step (b), the pressure is first raised to above 1 GPa, thetemperature then raised to bring the particles into a temperature andpressure region to produce plastic deformation of the particles, and thepressure thereafter raised to bring the particles into a region in whichthe particles are thermodynamically stable.
 13. A method according toclaim 11 or claim 12 wherein the abrasive product produced is sizereduced.
 14. A method according to claim 13 wherein the size reductiontakes place by crushing or milling.
 15. An abrasive product according toclaim 1 substantially as herein described with reference to any one ofthe Examples.
 16. A method according to claim 11 substantially as hereindescribed with reference to any one of the Examples.